Clipping Options & Observations
The stock OD2 configuration offers two switchable clipping choices--the traditional TS808 pair of silicon diodes (1N914 type), and a pair of red LED's as used in the highly touted (and, IMO, ludicrously expensive) Landgraff Dynamic Overdrive and numerous other "boo-teek" OD pedals. The MOSFET Conversion Kit offers the option of substituting a pair of BS170 MOSFETs for the red LED's, giving a different distortion character. The third position of the clipping toggle switch is what's known as a "diode lift", in that it lifts the clipping stage out of the circuit and passes the signal through unclipped.
In point of fact, you can modify the clipping configurations in a myriad of ways. Clipping is the term given to the process of inducing distortion into an audio signal by cutting off, or "clipping", a portion of it. Check out THIS ARTICLE
by effects guru R.G. Keen for a good overview of musical distortion. A pair of diodes--essentially one-way current gates--are a convenient way to induce signal clipping. They're installed in a circuit in such a way that, when a signal voltage is applied that exceeds the diode's turn-on or "threshold" voltage, the portion above that voltage value is dumped out of the signal path, usually to ground. Think of it as working like a safety relief valve does, only with voltage rather than pressure providing the device activation.
Because an audio signal is an AC signal, clipping diodes are nearly always installed in both polarity directions, so that both the rising and falling portions of the signal are clipped. The simplest and most common way to do this is just a pair of diodes of opposite polarity wired in parallel. This exactly how the stock OD2 clipping is set up, with a pair of 1N914 diodes and a pair of red LED's. A clipping stage set up this way--an equal number of the same type of diodes wired in parallel but opposing polarity--gives what is called symmetrical clipping, since the rising and falling audio signals are clipped in the same way. If different types or numbers of diodes are used on the two opposing polarity sides, the resulting clipping is called asymmetrical, because the rising and falling signals are processed differently. According to the conventional wisdom, symmetrical clipping gives rise to odd order harmonics that impart a raspier edge to the clipped tone. Asymmetrical clipping generates both odd and even order harmonics, with the latter having a smoother and somewhat more musical sound; the more asymmetrical the clipping, the more pronounced the even order harmonics become. Asymmetrical clipping has become very popular, due to its perceived/reputed quality of sounding more natural and "tube-like". But you really have to experiment and see what sounds best to you, because these are extremely subjective matters.
Two properties of the diodes themselves affect how the clipped tone sounds--the aforementioned threshold voltage, and the speed at which the diode turns on when the threshold is exceeded. Diodes that turn on quickly are said to give "hard" clipping, while slower ones clip more "softly". Hard clipping typically sounds more distorted and harsher in character than soft clipping. That can be good or bad, depending upon what tone you're going for. Different diode voltage thresholds determine how much of the signal is clipped, affecting both the degree of distortion and the volume of the clipped signal. A diode with large voltage threshold, like an LED (typically in the 2 - 2.5V range), clips far less signal than a low voltage threshold diode like a 1N34A germanium diode (~0.3V). Consequently, the output of the LED clipping would sound less distorted and very noticeably louder than the 1N34A clipped signal.
Things other than simple diodes can be used to generate signal clipping. The BS170's included in the MOSFET Conversion kit are a perfect example. Though these are a type of transistor and not a diode per se, their structure is such that they contain an internal diode called a "body diode" that gives them signal clipping capability when appropriately wired into the circuit. MOSFET clipping has become quite popular, due to its somewhat warmer, fuller sound compared to silicon diodes, and is used in some very popular pedals, including the Fulltone Fulldrive 2 MOSFET and the Hermida MOSferatu. I've used J201 JFET's in some other pedal circuits for clipping, and there are many other possibilities.
The bottom line is that you don't have to stick with the two stock choices or the optional MOSFETs when you build your OD2. Feel free to experiment. A good way to do this is to install sockets
in the diode connection eyelets on the PCB, so that you can try different types and combinations of diodes without needing to desolder/resolder the components. You can even solder multiple diodes together head to tail to make asymmetric combos. For example, one favorite of mine is a 1x3 1N914 diode array. Do keep in mind that you can use either the LED or MOSFET mounting locations but not both simultaneously. Uh-oh! The diode "lift" sounds just like the LED setting!?! What the....
No, you didn't do anything wrong. This apparent similarity in tone is for real, at least under many circumstances. The reason is that LED's have a high forward voltage threshold--typically in the 2.0 - 2.5V range--so it takes a BIG signal going through them before they clip any of it. So unless you have things cranked up pretty high--a hot signal into the pedal, the OD gain cranked up, and slammin' those strings pretty good--the LED's may very well be generating little to no signal clipping. And if that's the case, then they're functioning just like that diode lift position does, and the two settings sound the same. The fact that little to no signal is being lost via clipping is why they're both noticeably louder than the stock TS clipping setting.
But wait a second! If the diode "lift" isn't clipping any signal, how come you can still hear distortion? Well, that's because not all the distortion comes from signal clipping. The opamp gain itself adds a certain amount of distortion, plus the hotter output of the OD circuit may be generating distortion downstream in your signal chain. The preamp stage of your amplifier is getting pushed harder by the OD2's overdrive output and may be generating some degree of distortion just from that.
Because of this tonal similarity between the LEDs and the diode "lift", I prefer to build my OD2's with BS170 MOSFETs in the second clipping position. I like the warmer, smoother tone of the MOSFET clipping, and the middle "lift" setting on the clipping toggle gives you very close to the same tone as the LED's would anyway. So using the MOSFETs gives the pedal a broader range of different tones.
BTW, there's a simple way to see if your LED's are clipping, because when they do--they light up!! Not very much, and generally only for a fraction of a second at the initial attack of a note or chord, but if you remove the back of the pedal, turn it upside down, and darken the room, it's visible.